The present invention relates to a method and apparatus for determining the scrambling mode of a data signal such as a television signal in a cable television network, and providing corresponding scrambling mode data in a specific decoder-compatible format. The invention allows decoders to be compatible with an encoder of a different manufacturer or vendor.
Various techniques are known for scrambling television signals. In particular, an encoder can scramble a signal using modification (e.g., suppression or scrambling) of synchronization information and/or inversion of portions of the active video. Moreover, the scrambling mode can vary with time. For example, the level of suppression can vary between two or more levels every few seconds. The particular type of scrambling is known as the scrambling mode. Additionally, data which indicates the scrambling mode can be communicated with the scrambled signal itself to a decoder population for use by the decoders in descrambling the signal. For example, the scrambling mode data may be provided in the vertical blanking interval (VBI) of the television signal or in an audio subcarrier.
Accordingly, in order for a decoder to be able to recover and identify the scrambling mode data, it must have a priori knowledge of the scrambling mode data format. When the encoder and decoder are produced by the same manufacturer or vendor, or otherwise designed according to a common standard, there is no difficulty.
However, in practice, encoders and decoders which are currently available are produced by different manufacturers and may use a variety of different scrambling modes as well as scrambling mode data formats. Accordingly, cable television network operators and others are constrained to select specific encoder and decoder equipment which is compatible. This has various drawbacks for the network operators as well as the subscribers. For example, the network operators may not be able to employ the latest decoder features without updating the headend encoder, or to purchase new equipment at the most competitive price. Moreover, the network operator may be precluded from gradually updating the decoders in a network with a new type of decoder, if the new decoders are not compatible with the existing scrambling mode data format.
Accordingly, it would be desirable to have a system for detecting the scrambling mode of an analog television signal, and providing corresponding scrambling mode data in a format which is compatible with a specific decoder population.
The system should allow two or more different formats of scrambling mode data to be carried with a scrambled television signal to provide compatibility with two or more corresponding decoder populations.
The system should be able to detect modification (e.g., suppression or scrambling) of a synchronization pulse of a television signal as well as an inversion of an active video portion.
The system should further be able to detect dynamic changes in the scrambling mode of a television signal.
The system should not degrade the image quality of the television signal.
The present invention provides a system having the above and other advantages.